linux-kernel - Re: [PATCH v2 2/4] x86/vmware: Add a header file for hypercall definitions

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Message-ID: <20190827154422.GG29752@zn.tnic>
Date:   Tue, 27 Aug 2019 17:44:22 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
Cc:     linux-kernel@...r.kernel.org, pv-drivers@...are.com,
        linux-graphics-maintainer@...are.com,
        Thomas Hellstrom <thellstrom@...are.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        dri-devel@...ts.freedesktop.org, Doug Covelli <dcovelli@...are.com>
Subject: Re: [PATCH v2 2/4] x86/vmware: Add a header file for hypercall
 definitions

On Fri, Aug 23, 2019 at 10:13:14AM +0200, Thomas Hellström (VMware) wrote:
> +/*
> + * The high bandwidth out call. The low word of edx is presumed to have the
> + * HB and OUT bits set.
> + */
> +#define VMWARE_HYPERCALL_HB_OUT						\
> +	ALTERNATIVE_2("movw $" VMWARE_HYPERVISOR_PORT_HB ", %%dx; rep outsb", \

Hmm, that looks fishy:

This call in vmw_port_hb_out(), for example, gets converted to the asm
below (I've left in the asm touching only rDX).

# drivers/gpu/drm/vmwgfx/vmwgfx_msg.c:160:              VMW_PORT_HB_OUT(
#NO_APP
        movzwl  0(%rbp), %edx   # channel_20(D)->channel_id, channel_20(D)->channel_id

	...

        sall    $16, %edx       #, tmp172
        orl     $3, %edx        #, tmp173

this is adding channel_id and flags:

                        VMWARE_HYPERVISOR_HB | (channel->channel_id << 16) |
                        VMWARE_HYPERVISOR_OUT,

the $3 being (VMWARE_HYPERVISOR_HB | VMWARE_HYPERVISOR_OUT).

        movslq  %edx, %rdx      # tmp173, tmp174

Here it is sign-extending it.

#APP
# 160 "drivers/gpu/drm/vmwgfx/vmwgfx_msg.c" 1
        push %rbp;mov %r8, %rbp;# ALT: oldinstr2        # bp
661:
        movw $0x5659, %dx; rep outsb

And now here you're overwriting the low word of %edx. And now it
contains:

0x[channel_id]5659

and the low word doesn't contain the 3, i.e., (VMWARE_HYPERVISOR_HB |
VMWARE_HYPERVISOR_OUT) anymore. And that's before you do the hypercall
so I'm guessing that cannot be right.

Or?

-- 
Regards/Gruss,
    Boris.

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